Batteries comprising electrochemical cells serve widespread as mobile low-voltage direct current electricity sources. Present-day most frequently used secondary (rechargeable) batteries are lithium ion batteries. Such kind of battery can be built e.g. using transition metal oxide that contains lithium cations in the first electrode and crystalline or amorphous carbon in the second electrode as active materials, with a lithium salt as an electrolyte. The charging is then performed via connecting the first electrode to the positive pole of an external voltage source and the second electrode to the negative pole of the external voltage source, what results in an oxidation of the transition metal in the first electrode polarized as anode, deintercalation of the Li cations from the anode, their electromigration through the electrolyte, their reduction on the second electrode polarized as cathode, and in the intercalation of the formed electrically neutral Li atoms in the carbon matrix of the second electrode. When discharging this battery by connecting its poles to an external electrical resistance, the first electrode as the positive pole of the battery consumes electrons coming through the external circuit from the second electrode that represents the negative pole of the battery. The first electrode in the discharging battery is called cathode, because there takes place an electrochemical reduction of the transition metal to its lower valence, accompanied by intercalation of the Li cations form the electrolyte. The second electrode in the discharging battery is assigned, according to the electrochemical convention, as anode, because there takes place an electrochemical oxidation of the intercalated Li atoms, releasing electrons in the external circuit and Li cations in the electrolyte.
In batteries for the most of consumer devices, energy storage and operating time are the keys, so the more the better. Especially for mobile applications like cell phones, electric vehicles (EV) or hybrid vehicles (HEV), higher power and energy densities are required.
In this regard, electrochemically active materials play an essential role. For the use in rechargeable batteries, only highly reversible redox systems are applicable. For allowing high energy density, low molecular weight compounds exchanging high number of electrons per molecule or polymers with low molecular weight building units exchanging high number of electrons per building units are preferable, most preferably those showing, moreover, high redox potential. Among further (often somewhat contradictory) requirements on electroactive materials for batteries can be named their safety, intrinsic chemical stability and low aggressiveness, good mechanical properties, easy processability, environmental tolerability, abundance of necessary raw materials and low manufacturing cost. Solid or gel-like electroactive materials are preferred.
As an alternative to transition metal based inorganic redox-active materials, electroactive organic materials offer potential advantages in their light weight, low-temperature processability, raw material accessibility and lowering the environmental burden by absence of toxic metals like nickel. Examples of organic electroactive materials recommended for use in batteries are nitroxide compounds reported e.g. in US2004/115529 or tetracyanoquinodimethane (TCNQ) reported in Japanese patent application S57-210567. Yet, the performance of organic electroactive materials, especially in terms of their electrical properties, energy density, reversibility and stability, has to be significantly improved.
The object of the present invention is to provide batteries with improved energy density. Another object of the invention is to provide electroactive organic materials with improved energy density and sufficient stability and reversibility for their use in rechargeable batteries. Yet another object of the invention is to provide new compounds enabling the inventive electroactive organic material and batteries comprising it.